A lithium-ion battery comprises at least one negative electrode, one positive electrode, a separator and an electrolyte. The electrolyte consists of a lithium salt dissolved in a solvent which is generally a mixture of organic carbonates, in order to have a good compromise between viscosity and dielectric constant.
Among the salts which are the most widely used is lithium hexafluorophosphate (LiPF6), which has many of the numerous qualities required, but has the disadvantage of degrading in the form of hydrofluoric acid gas. This poses safety problems, in particular in the context of the use in the near future of lithium-ion batteries for particular vehicles.
Other salts have therefore been developed for providing electrolytes of Li-ion batteries, and in particular LiTDI (lithium 1-trifluoromethyl-4,5-dicarbonitrileimidazolate) and LiPDI (lithium 1-pentafluoroethyl-4,5-dicarbonitrileimidazolate), as is taught in document WO 2010/023413. These salts have the advantage of having fewer fluorine atoms and of comprising strong carbon-fluorine bonds in place of the weaker phosphorus-fluorine bonds of LiPF6. In addition, these salts have very good conductivities of about 6 mS/cm, and a very good dissociation between the imidazolate anion and the lithium cation.
Document WO 2010/023413 proposes several synthesis routes for the production of these pentacyclic anions, one of which consists of the condensation of diaminomaleonitrile (DAMN) with an acid derivative such as a fluorinated acid anhydride, followed by proton/lithium exchange. The salt obtained is then purified so as to achieve a composition which is optimal with respect to its performance levels within an electrolyte for a Li-ion battery.
However, the salts produced after the purification step of WO 2010/023413 poses a problem in the application for batteries. The common methods of analyses have not made it possible to identify and/or quantify the organic and inorganic impurities, present in the salts, that can be detrimental to their use as an electrolyte in batteries.
A first subject of the present invention is a pentacyclic anion salt composition which does not have the abovementioned drawbacks.
A subject of the present invention is also the use of this composition as an electrolyte in batteries.
A subject of the present invention is also a production process for obtaining the composition according to the first subject.
The invention provides the threshold of content of ionic and nonionic compounds present in the pentacyclic salt composition which is suitable for the performance levels of the electrolyte in a battery, in particular the quality of establishment of the SEI (Solid Electrode Interface) and also the retention of capacity of the battery during charge-discharge cycles.